There are many photo editing tools out there and typically you will get one free with your camera or scanner. And you would like a great one for installing on your computer try Google's Picasa. But, let's say you are at the office, at you school and you need to edit a photo right now and you don't have Picasa, or another photo editor installed. And you don't have admin rights to install Picasa, well then go to PicNik at http://www.picnik.com/. The is a free online web-based photo editing tool. You can even use it without an account but sign up for one, it is free. And PicNik is VERY easy to use. You can use it and direct your students to it. If you have a Flickr account it can connect directly to it.
I found that I could easily do the following useful things:
Remove red-eye from a photo
Take a photo using my Web Cam
Crop that photo
Resize the photo
Adjust the brightness
Save it to me computer or email it to a friend
And a lot more features
One thing I would find very useful is that often students will send me or post a photo to our course site but they have not resized it down to fit within a screen. This is normally due to the new digital cameras using resolutions optimized for printing but not screen viewing and they don't know how to resize it down, or crop it. And often when I ask them if they have a photo editor on their computer, they have no idea. So I am going to send them to the PikNik site.